1. Field of the Invention
This invention relates in general to subsea running tools and, in particular, to sensing the relative turns and relative displacement of a subsea running tool at mud line and sub mud line levels.
2. Brief Description of Related Art
In subsea operations, a surface platform generally floats over an area that is to be drilled. The surface platform then runs a drilling riser that extends from the surface platform to a wellhead located at the sea floor. The drilling riser serves as the lifeline between the vessel and the wellhead as most drilling operations are performed through the drilling riser. As devices are needed for the well, such as casing hangers, bridging hangers, seals, wear bushings, and the like, they pass from the surface of the vessel on a running string through the riser, through the wellhead and into the wellbore. Weight, rotation, and hydraulic pressure may be used to place and actuate these devices. Because of this, it is important to know with some specificity the relative number of turns and displacement of the running tool in the subsea environment. Knowing this information allows operators to know that the device has reached the appropriate position in the wellbore and properly actuated. Typically, this is accomplished by monitoring the number of running string turns and displacement of the running string at the surface platform.
Because surface platforms float over the subsea wellhead, they are subject to the effects of ocean currents and winds. Despite attempts to anchor the riser to the sea floor, ocean currents and winds will push surface platforms such that they do not remain completely stationary over the wellhead. In addition, the riser itself is subject to movement due to ocean currents. Because of this, the riser will not remain truly vertical between the wellhead and the surface platform. Instead, the riser will “curve” in response to the position of the vessel in relation to the wellhead and the effects of the current on the unanchored riser sections extending between the ends of the riser string anchored at the surface platform and at the wellhead. As locations in deeper water are explored, the problem becomes exacerbated.
As the riser curves, the running string passing through the riser will contact the riser rather than remaining coaxial within the riser. At the locations where the running string contacts the riser wall, the running string becomes anchored, and transmits some of the operational weight and torque, applied by the surface platform to the running string, from the running string to the riser. Thus, the actual torque and weight applied to the device in the wellbore is less than the total torque and weight applied at the surface platform. This difference within the relative number of turns and displacement of the running tool compared to the number of turns and running string displacement at the surface.
In addition, the difference in the number of turns and displacement applied at the surface and the number of turns and displacement at the running tool may be realized because of the length of the running string. The running string may extend thousands of feet through the riser between the wellhead and the surface. When turned, the segments of the running string may twist relative to one another, such that a portion of each turn is absorbed by the running string. Similarly, some axial displacement is absorbed by displacement of running string segments relative to one another. Thus, turns and displacement applied at the surface may not translate to an equal displacement or number of turns at the running tool at the wellhead. Therefore, there is a need for a method and apparatus for sensing number of turns and displacement of the running tool at a mud line and sub mud line level while landing, setting, and testing subsea wellhead devices with a running tool.